The basis for uplink (UL) and downlink (DL) scheduling is dynamic scheduling. In a long term evolution (LTE) wireless communication system, scheduling information is transmitted during transmission timing intervals (TTIs) to a wireless transmit/receive unit (WTRU) via a physical downlink control channel (PDCCH). It has been agreed by radio access network (RAN) working groups (i.e., RAN2) to support semi-persistent scheduling for the DL and the UL in long term evolution (LTE). For semi-persistently scheduled wireless transmit/receive units (WTRUs) in a transmission time interval (TTI), a DL/UL grant does not need to be sent for initial data transmission. The only exception is when an evolved Node-B (eNB) wants to override the persistent resource assignment, which by definition should be infrequent. Otherwise, the sole purpose of a DL/UL persistent resource assignment is lost. As an optimization for voice over Internet protocol (VoIP), persistent scheduling is used for both DL and UL, where the resource for the initial transmissions is persistently allocated and the resources for the hybrid automatic repeat request (HARQ) retransmissions is dynamically allocated.
For the DL, the HARQ Process identification (ID) of a re-transmission must be specified because the same cannot be inferred from the TTI in which the re-transmission occurs, (due to the asynchronous nature of DL re-transmissions). For dynamic scheduling, this is achieved via DL grant signaling. For semi-persistently scheduled WTRUs in a TTI, a DL grant is not sent for initial transmissions, and any subsequent adaptive re-transmissions require a DL grant which explicitly indicates the HARQ Process ID for the re-transmissions.
A solution includes both the WTRU and the eNB to assume that the next available HARQ process-ID is used for a persistently scheduled DL transmission. However, this procedure would not be robust enough in the presence of errors.
When HARQ processes are reserved for persistent transmissions, the HARQ processes left for dynamic transmission are limited. As such, it is not efficient to reserve several HARQ processes only for persistent transmissions if they finish the transmission successfully and cannot be used by other services for a long time.
Therefore, there exists a need for a method to alleviate the above inefficiencies and concerns.